Positive update

Implement positive update.

Positive update, updates when:
-poem is created (matches created_at)
-poem snap count increases (matches updated_at)
-NOT when poem snap count decreases
-NOT when any other attribute is changed


A way of ordering poems with the ones that most recently were appreciated or created comes to the top. 

Right now, if a poem is unsnapped the previous positive update (before the snap by a user that was unsnapped) is not restored.